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ALEXANDRIA, Virginia (CNN) -- The FBI agent who arrested and interrogated al Qaeda conspirator Zacarias Moussaoui a month before the September 11, 2001, attacks testified Monday that he believed at the time that Moussaoui was a terrorist intent on hijacking an airplane.

But agent Harry Samit told jurors, "What I believed and what I could prove are two different things."

Samit, assigned to the agency's field office in Minneapolis, Minnesota, blamed FBI headquarters for having "obstructed" the Moussaoui probe, which the government now portrays as the lost opportunity to unlock clues to the attacks.

Samit testified that supervisors at FBI headquarters challenged the identification of Moussaoui, a French citizen, assigning agents to check his name against listings in the Paris telephone book.

Samit said he and his colleagues considered the request a waste of time.

Supervisors at FBI headquarters also repeatedly denied his requests to obtain criminal or national intelligence warrants to search Moussaoui's belongings, Samit testified.

Moussaoui's belongings contained contact numbers for a key September 11 attack planner in Germany and short-bladed knives such as the ones the hijackers used to commandeer four jets. But they weren't searched until after the attacks.

U.S. eyes possible charges

Samit's cross-examination was delayed a week after prosecutors disclosed that government lawyer Carla Martin e-mailed commentary, opening statements and witness testimony to a half-dozen scheduled aviation security witnesses. (Who is Martin and did she harm the case? -- 1:52)

U.S. District Judge Leonie Brinkema found that Martin violated her court order instructing witnesses to avoid following court proceedings or discussing them with each other until they had testified. She agreed Friday to allow other, "untainted" witnesses to testify for the government and will screen them in a closed-door hearing Tuesday morning.

Law enforcement sources said the Justice Department is exploring possible charges against Martin but no decisions have been made. (Full story)

In Monday's rigorous, five-hour cross-examination, Moussaoui defense attorney Edward MacMahon built on a foundation laid by a Justice Department inspector general's probe and congressional joint inquiry into the investigative shortcomings that preceded the attacks.

The defense attorney's questions brought to light the first public displays of e-mail detailing what MacMahon called a "bureaucratic bind."

Samit also revealed what he admitted in hindsight were errors of his own. They included opening the Moussaoui case as an intelligence investigation, not a criminal one, and failing to ask certain questions of the suspect.

"Although I did create problems for myself, I had no other choice," Samit said.

Agent said he didn't believe Moussaoui

Samit questioned Moussaoui on August 16 and 17, 2001, when the French native of Moroccan descent was initially in custody for an immigration overstay. Samit and an immigration officer arrested Moussaoui after tips from a Minnesota school that offers students training on a Boeing 747 flight simulator.

As he had March 9, Samit again told the jury that Moussaoui said he was in flight school for fun and intended to visit New York.

Samit said he believed Moussaoui was lying. His traveling companion, a friend from Oklahoma named Hussein al-Attas, had told agents Moussaoui talked about "jihad," or holy war, and his approval of martyrdom.

Moussaoui revealed under Samit's questioning that he spent two months in Pakistan just before his arrival in the United States in February 2001. He claimed he stayed in the Karachi area and had been looking for a wife.

Although Pakistan is a gateway to Afghanistan, Samit conceded he never asked Moussaoui directly if he had been to Afghanistan or attended a terrorist training camp.

Samit said he considered Moussaoui "the most serious" matter and composed a 26-page e-mail laying bare his fears.

The August 18 memo, copied to at least 50 people at FBI headquarters, said Moussaoui's evasiveness, his religious views, his possession of knives and his bulking up at a gym gave the Minneapolis office "cause to believe" that Moussaoui and "others yet unknown others were engaged in a "conspiracy to commit a terrorist act."

Samit needed to show either "probable cause" of a crime to obtain a criminal search warrant or evidence that Moussaoui was here at the behest of a "foreign power" to pursue an intelligence warrant.

Agent: Probe 'shut down'

FBI headquarters first rejected the criminal warrant August 21. A discouraged Samit would never seek another, even after he learned Moussaoui had made false statements for which he could be prosecuted.

Instead, he said he focused on gathering information to support an intelligence warrant.

Britain, where Moussaoui lived for eight years, was slow to respond to FBI requests for more information, and did not reply until after September 11.

France, however, communicated in late August possible Moussaoui connections to Chechen rebels backed by Osama bin Laden.

The supervisor Samit saw as his biggest obstacle, Mike Maltbie, later complained that the Minneapolis agents were getting people "spun up" about Moussaoui.

Samit said Maltbie preferred to deport Moussaoui to France, where he could be searched more freely. Then Maltbie quibbled over which government would pay for the plane tickets, Samit said.

Samit said he was effectively "shut down" by FBI headquarters, and his office was "denied every tool at the division's disposal."

He never made another request for an intelligence warrant and was forbidden from contacting local federal prosecutors for a criminal one.

Samit later told the inspector general he felt the Washington supervisors performed "misconduct of the most serious kind."

Prosecutors contend the lies Moussaoui told Samit, covering up al Qaeda's conspiracy to hijack and crash planes into prominent buildings, contributed to nearly 3,000 murders September 11. As a result, the government says, Moussaoui deserves the death penalty.

Prosecutors argue that if Moussaoui had alerted law enforcement, the FBI could have identified 11 of the 19 hijackers, and the Federal Aviation Administration could have stopped some of them at airport gates by banning short knives or adding conspirators' names to a "no-fly" list.